Microwave assisted content analyzer

ABSTRACT

A method and associated apparatus are disclosed for microwave assisted content analysis. The method includes the steps of applying microwave radiation to a sample while concurrently rotating the sample while the microwave radiation is being applied and while concurrently weighing the sample and while measuring the temperature of the sample and controlling or moderating the microwaves applied to the sample based upon the measured temperature.

FIELD OF THE INVENTION

[0001] The present invention relates to the content analysis ofmaterials. In particular, it relates to the use of microwave radiationto assist in moisture content analysis and related techniques inmaterials with compositions that are less responsive to the effects ofmicrowave radiation.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to the content analysis ofmaterials. Because the contents or compositions of materials give thosematerials their particular properties, knowing such content can beparticularly important for purposes of making, using, processing, orotherwise dealing with various materials.

[0003] The techniques used to analyze the contents of various materialsare almost endless. The present invention relates particularly tomoisture content analysis, and has secondary, but important aspectsrelated to the measurement of other materials in a sample, such as (butnot limited to) fats and oils, and volatiles other than water. Foods area particularly useful example of materials for which knowledge ofmoisture content is quite valuable, and some cases required (e.g. manystatutory and regulatory schemes apply to the content and labeling offood products). Accordingly, a number of techniques have been developedfor measuring the moisture content of materials and particularly themoisture content of foods. In a conventional technique referred to asgravimetric moisture analysis, a sample is weighed, dried, and thenreweighed. Comparing the two weights gives an absolute weight loss,which, if the material has not degraded, can be used to calculate andexpress the moisture content as a percentage.

[0004] Heat (often convection heating) is typically used to acceleratethe removal of moisture from materials in gravimetric analysistechniques. The use of a typical oven or a hotplate for such purposes,however, raises a number of associated problems or difficulties. First,if heat tends to degrade the material, rather than merely drive off themoisture, the technique is unsuitable for moisture content analysis ofthat material. Additionally, because heated samples create air currentsthat can affect the output readings of sophisticated balances, heateddrying techniques require relatively long waiting periods betweenmeasurements, with the measurement having to be repeated until the driedweight remains constant.

[0005] In more recent developments, microwaves have been used to drivemoisture from samples and thus assist in the gravimetric analysis ofsuch materials. This technique has worked extremely well in a number ofcircumstances, and appropriate moisture analyzers using microwavetechniques have been available for some years, e.g., U.S. Pat. Nos.4,438,500 and 4,457,632.

[0006] Furthermore, various improvements continue to make the use ofmicrowave assisted techniques more feasible for more and more difficultmaterials. Again using food as the example, items with relatively highfat content (such as cheese and other dairy products) or low moisturecontent (e.g., powdered milk) can be difficult to analyze (for moisturecontent) using microwave techniques because they tend to cook or degradewhen heated, rather than simply give up moisture.

[0007] Exemplary improvements in microwave techniques for such purposesare set forth in co-pending and commonly assigned applications Ser. No.09/398,129 filed Sep. 17, 1999 for “High Efficiency Cavity Design,” Ser.No. 09/397,825, also filed Sep. 17, 1999 for “High Efficiency AirShield”; Ser. No. 09/398,130 filed Sep. 17, 1999 for “Method forCorrecting Weight Measurement Errors During Microwave Heating”; and Ser.No. 09/156,086 filed Sep. 17, 1998 for “Method and Apparatus forMeasuring Volatile Content.” All describe improvements in methods,apparatus, and techniques for moisture and related content analysis ofmaterials that are otherwise difficult to handle usingmicrowave-assisted techniques. The contents of each of these pendingapplications are incorporated entirely herein by reference.

[0008] The most effective microwave heating of various materials tendsto depend upon the presence of free moisture in the material to beheated. Thus, for samples with little or no free water, microwaveheating is difficult or ineffective. In this regard a microwaveapparatus incorporating a microprocessor can monitor the drying curve(weight loss v. time) of a sample and can predict the final dry weight(or moisture content) based on an initial portion of the drying curve.Such analyses usually may be conducted in about two to three minutes forsamples that contain free water

[0009] Nevertheless, some or all of the moisture present in low moisturecontent materials, such as dried milk powder and detergents, tends to bebound water (e.g. water of hydration) or for other reasons resistsremoval by microwave radiation. In such cases, applying microwaveradiation solely to the sample tends to be generally unsatisfactory andoften unsuccessful, particularly when such materials tend to burn ordegrade before all of the moisture (bound or free) has been removed.

[0010] Accordingly, techniques have been incorporated in which a sampleis placed on a material that absorbs microwaves and becomes heated inresponse to those microwaves. U.S. Pat. No. 4,681,996 is an example ofone such technique. As set forth therein, the goal is for thethermally-responsive support to conductively heat the sample to releasethe bound water. Theoretically, a truly synergistic effect should beobtained because the thermally heated support in turn heats the sampleto remove bound water while the free water responds to, and is removedby, the direct effect of the microwaves upon it.

[0011] Additionally, when non-polar solvents are present with bound orfree water in a material to be analyzed for volatiles, they are likewisevolatilized by the thermal heat from the support, while free water(which may have been thermally freed from a bound form), is vaporized bythe microwave radiation. Thus, volatiles are quickly removed from thesample whether the volatiles are bound water, free water, other polarmaterials or non-polar compounds.

[0012] The techniques set forth in the '996 patent, however, are lesssuccessful in actual practice. As one disadvantage, the supportsdescribed therein are self-limiting in temperature response tomicrowaves, and thus pads of different composition are required fordifferent desired temperatures. As another, the device and method failedto successfully address the differences in measured weight caused whenair density gradients affected the internal balance. As a thirddisadvantage, no technique other than the self-limitation of the pad wasavailable for measuring or controlling the temperature of the sampleduring the overall process, and the predictability of the pad'stemperature was erratic. As known to those familiar with contentanalysis, certain standardized drying tests are based on a“loss-on-drying” measurement in which a sample is heated to andmaintained at a specified temperature for a specified time. The weightloss under such conditions provides useful and desired information,provided the test is run under the specified conditions. Thus, absenttemperature control, microwave techniques cannot be used forloss-on-drying analysis.

[0013] Accordingly, it remains a goal of this industry to continue toimprove the apparatus and techniques under which content analysis can becarried out using microwave-assisted techniques, and can be done so formaterials that are otherwise less responsive, or even non-responsive, tomicrowave radiation.

OBJECT AND SUMMARY OF THE INVENTION

[0014] Therefore, it is an object of the invention to provide animprovement in microwave-assisted techniques for content analysis.

[0015] The invention meets this object with a method ofmicrowave-assisted content analysis. The method includes the steps ofapplying microwave radiation to a sample and susceptor pad whileconcurrently rotating the sample and pad while the microwave radiationis being applied, while concurrently weighing the sample, whilemeasuring the temperature of the sample and pad, and while (if necessaryor desired) moderating the microwaves being applied in response to themeasured temperature.

[0016] In another aspect, the invention is an apparatus formicrowave-assisted content analysis. The apparatus includes a source ofmicrowave radiation, a cavity in microwave communication with saidsource, a balance with at least its pan (or pan equivalent) in saidcavity, and means for rotating said balance pan within said cavity whileconcurrently measuring the weight of a sample on said pan.

[0017] In yet another aspect, the invention is a susceptor for microwaveassisted content analysis that complements the method and apparatus. Thesusceptor is a pad formed of ablend of a first material dispersed in asecond material, the first material being a material that generates heatin response to microwave radiation, and the second material beingnonresponsive to microwave radiation, but conductively heated by saidfirst material so that exposing said susceptor to microwave radiationevenly heats said pad and a sample on said pad.

[0018] These and other objects and advantages of the present inventionwilll become clearer when taken in conjunction with the followingdetailed description and the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a perspective view of an apparatus according to thepresent invention;

[0020]FIG. 2 is a perspective view of the apparatus according to thepresent invention with the cover removed and particularly illustratingthe balance pan and the means of rotation;

[0021]FIG. 3 is a view of the balance pan and the means for rotating ittaken from FIG. 2 in simplified fashion;

[0022]FIG. 4 is a cross sectional view of the rotating balance panaccording to the present invention; and

[0023]FIG. 5 is a susceptor according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] In the first aspect, the invention is a method ofmicrowave-assisted content analysis that comprises applying themicrowave radiation to a sample on a susceptor pad while concurrentlyrotating the sample while the microwave radiation is being applied,while concurrently weighing the sample, while measuring the temperatureof the rotating sample and pad, and moderating the applied microwaveradiation in response to the measured temperature. In preferredembodiments, the method comprises maintaining the balance accuracywithin about 10 milligrams (mg) while concurrently weighing and rotatingthe sample. In more preferred embodiments the method comprisesmaintaining the balance accuracy within 1 milligram, and in the mostpreferred embodiments, the method comprises maintaining the balanceaccuracy within 0.1 milligram.

[0025] Preferably, the method comprises rotating the sample on a samplepad—most preferably on the susceptor described below—at a speed ofrotation that is sufficient to maintain an equilibrium temperature, or aequilibrium rate of temperature change, in a sample on the pad beingrotated (or in the temperature of the sample and pad), but less than aspeed at which the sample would become dislodged from the pad (usuallyby centrifugal force) or at which the desired accuracy of the balancewould be reduced. Generally speaking, rotating the balance at higherspeeds lowers the balance accuracy. Accordingly, in more preferredembodiments the method comprises rotating the sample pad at a speed ofless than about 30 revolutions per minute (rpm), and in the mostpreferred embodiments the method comprises rotating the sample on thesusceptor at a speed of between about 5 and 20 revolutions per minute.

[0026] In a manner consistent with good practices in microwave assistedchemistry and content analysis, the microwaves are preferably generatedat the source, transmitted through a waveguide that is in communicationwith the source, and then transmitted from the waveguide to a cavitythat contains the sample.

[0027] In order to control the temperature of the sample and pad, theinvention incorporates the infrared temperature detection technique setforth in previously-incorporated Ser. No. 09/156,086, and the microwavepower moderation techniques set forth in Ser. No. 09/063,545(incorporated by reference below), or alternatively the microwavemoderation technique set forth in commonly assigned U.S. Pat. No.5,796,080 which is likewise incorporated entirely herein by reference.

[0028]FIG. 1 illustrates a commercial embodiment of the presentinvention broadly designated at 10. In the illustrated embodiment, themicrowave content analyser 10 includes a cover portion 12 and a baseportion 13 which together rest on an operative pedestal 14 that containssome of the electronics as will be described with respect to FIG. 2.Because the content analyser 10 of the present invention can usefullyincorporate a processor for carrying calculations on the measurementsmade by the content analyser 10, the analyser 10 includes a keypad 15and display 16 for input and output of appropriate data or instrumentsettings. A paper tape drive 17 is also included and provides a printout of appropriate results. Additional features of the invention and thecontent analyser in particular are set forth in those referencespreviously incorporated herein.

[0029]FIG. 2 is an illustration of the lower portion 13 of the contentanalyzer 10 with the cover portions removed. The content analyzer 10includes a source of microwave radiation shown as the magnetron 20.Although a magnetron is a good source in terms of its combination offunction and relative cost, those familiar with the propagation ofmicrowaves will recognize that the source could also consist of aklystron, a solid state device or a switching power supply. In thisregard, the use of a switching power supply is set forth in commonlyassigned U.S. patent application Ser. No. 09/063,545 filed Apr. 21, 1998for “Continuously Variable Power for Microwave Assisted Chemistry”, thecontents of which are incorporated entirely herein by reference. Awaveguide (which is covered by other elements in the orientation of FIG.2) connects the source 20 of microwave radiation with the cavity. Thecavity is likewise described in more detail in other co-pendingapplications that have already been incorporated, particularly Ser. No.09/396,129. A balance 21 has at least its pan 22 positioned in thecavity. The pan 22 does not need to be physically solid. In a preferredembodiment illustrated in FIGS. 2 and 3, the pan is formed of a circularsupport portion 23 with several struts 24 maintaining the support 23 onthe vertical shaft 25 of the balance 21. In preferred embodiments, thebalance is a force restoration balance with an accuracy of ±0.1 mg.

[0030]FIG. 2 also illustrates that the portion of the balance that is inthe cavity does not interfere with the propagation of microwaves in thecavity or the modes of microwaves in the cavity while the analyser is inoperation. Accordingly, in preferred embodiments, the balance pan 22including the struts 24, the circular support 23 and the appropriateportions of the shaft 25, are formed of materials that do not interferewith or absorb microwaves. Stated differently, they are preferablyformed of materials that are transparent to microwave radiation.Generally speaking, such materials will consist of appropriate polymersor other non-metallic materials of which many are well known to those ofordinary skill in the art, and can be selected without undueexperimentation. To the extent that more portions of the balance can beformed of materials that do not interfere with microwaves, or can beotherwise shielded from microwaves, those portions can likewise extendinto the cavity. In present embodiments, it appears to be most useful,however, to have only the balance pan 22 and a small portion of theshaft 25 extend into the cavity, with the remainder of the balance beingshielded from the cavity.

[0031]FIGS. 2 and 3 further illustrate that the rotation of the shaftand the pan 22 are best accomplished in preferred embodiments by the useof an air pump 26 and its associated motor 27. A hose 30 carries astream of air to its terminal portion 31 adjacent to an impeller 32 thatis coaxially mounted on the shaft 25. The airflow from the terminalportion 31 of the hose 30 turns the impeller 32 which, being fastened tothe shaft 25, rotates the balance pan 22 in a desired fashion. It willbe understood that there are a number of appropriate engineeringsolutions for rotating the pan 22 with the desired speed and whilemaintaining the desired balance accuracy. These will be recognized bythose of skill in this art and can be designed and built without undueexperimentation and are equivalents of the design illustrated herein,which should be considered illustrative, rather than limiting, of themethod in which the pan 22 can be rotated. In general, however, the airand impeller drive of the preferred embodiment provides a non-contacttechnique that avoids interfering with the balance. Magnetic drives willwork, but must be carefully shielded from the microwave radiation in thecavity. In turn, if a magnetic drive is used, it must be shielded fromthe balance if the balance is sensitive to a magnetic field. Mechanicaldrives can be used, but are more difficult to integrate with theweighing functions of the balance.

[0032] The apparatus content analyzer 10 of the present invention canalso include an appropriate microprocessor. FIG. 2 illustrates aprocessor at 34 mounted on an appropriate board (or card) 35. Theprocessor 34 can be any appropriate processor that can carry out theintended functions. Specifically, the processor 34 typically receivesoutput information from the balance 21. By comparing the output from thebalance 21 against an internal clock, is able to calculate the weightloss over a time as a sample is dried in the content analyzer 10. Thereare several well-known relationships between drying time and moisturecontent that the processor can use to determine the end point of a givensample. In a conventional method, the processor 34 can identify when theweight has remained constant for a sufficient period of time to indicatethat the sample has no more moisture to lose and that the run can beended. In other techniques (e.g. U.S. Pat. No. 4,438,500), the processorcan track the weight loss along a generally well understood algebraiccurve, and predict an appropriate end point even before the sample hascompletely dried. In a simple, but effective technique, the sample isheated for a predetermined and fixed time. This fixed-time techniqueappears to be particularly satisfactory for low moisture-contentsamples. These and other techniques for determining the end point ofcontent analysis, and the manner of using a processor to assist in theend point determination, are generally well understood in the art andwill not be discussed in detail herein. Those of ordinary skill in thisart can produce such programs and select such processors without undueexperimentation. Generally speaking, however, the processor 34 will bethe same as or very similar to those used in personal computers.

[0033] The other components of the card 35 can be selected as desired ornecessary, and will not be discussed in detail, other than to note thatthe card 35 includes appropriate input and/or output ports (e.g. serial,parallel, USB) 36, 37, and 40.

[0034]FIG. 2 also illustrates a portion 41 of the overall housing, aswell as a shielding portion 42 that separates certain of the electronicdevices such as the card 35 from the balance 21 and the source 20. Thesephysical structures are generally conventional, and can be formed of anyappropriate material that does not otherwise interfere with theoperation of the content analyzer 10. Appropriate materials and theirdesign and manufacture are well understood in the art and can beproduced by those of ordinary skill without undue experimentation.

[0035]FIG. 4 is a cross sectional view of one embodiment of theinvention showing the manner in which the balance pan 22 is supportedfor rotation.

[0036]FIG. 4 is a cross-sectional view of the balance pan and rotationmechanism according to the present invention. As in FIGS. 2 and 3, thebalance pan is broadly designated at 22, with the circular support 23and struts 24 also illustrated along with the shaft 25. In FIG. 4, thebalance is again broadly designated at 21 with its mechanism beingindicated by the shaded rectangle 44. The turbine or impeller 32 ismounted on the shaft 25. In order to adapt the balance pan for rotation,the balance includes a balance stem adapter 46, which is connected bysetscrews 47 and 50 to both the balance mechanism 44 and the impeller32. Two respective sets of bearings 51 and 52 provide the support forthe impeller to rotate while the balance stem adapter 46 keeps itmaintained with respect to the rest of the balance mechanism 44 toprovide the appropriate weight measurements.

[0037] In preferred embodiments, the balance pan 22 includes the driptray illustrated at 53, which catches appropriate liquids that may dropfrom the sample on the balance pan and prevents them from interferingwith the rotation or weight measurement functions of the balance 21. Inpreferred embodiments, the balance stem 25 is formed of a material thatdoes not otherwise interfere with or respond to the microwave radiationin the cavity. Preferred materials include any polymer that offers therequired structural integrity and does not absorb microwave radiation.Such materials are well known in this art and can be selected by thoseof ordinary skill without undue experimentation. FIG. 4 also illustratesa balance cover 54, a circuit board 55 for the balance, and variousinput/output or power ports 56 for the balance.

[0038]FIG. 5 illustrates in schematic fashion a susceptor 60 accordingto the present invention. In preferred embodiments, the susceptor 60 isformed of a first material dispersed with a second material. The firstmaterial is the material which generates heat in response to microwaveradiation and the second material is generally less responsive ornon-responsive to microwave radiation, but conductively heated by thefirst material so that exposing the susceptor 60 to microwave radiationevenly heats the pad and any sample on the pad. In preferredembodiments, the susceptor 60 is a circular disc approximately 4⅜ inchin diameter by about {fraction (1/10)} inch in thickness. These are, ofcourse, exemplary measurements (and a circle is an exemplary shape) andare not limiting of the susceptor of the present invention.

[0039] In the most preferred embodiments, the susceptor 60 is formed ofthree materials. In such embodiments, the third material is similar, butnonidentical to the first material. The third material is similar inthat it absorbs microwaves and generates heat in response, but in asomewhat different manner than does the first material. To date, it hasbeen empirically observed that a combination of two such different, butabsorbing materials provides far more even heating of the susceptor—andthus far more even heating of the sample—than does the use of onematerial alone. The absorbing materials are preferably present in theminimum amount that provides the pad with a rapid and uniform responseto microwave radiation. In the size described above the pad 60 istypically formed of a combination of 4 grams (g) of graphite, 5 g ofsilicon carbide (SiC), 33 g of silicone resin and a few drops of ahardening catalyst for the silicone resin. This corresponds to about 12percent by weight silicon carbide, about 10 percent by weight ofgraphite, and about 78 percent by weight of the silicone rubber.

[0040] Those of ordinary skill in the art will recognize that in termsof the sole function of responding to microwaves, solid silicon carbidecan be used as a susceptor material as can solid graphite, or even othermaterials such as garnet (e.g., silicate materials). Each of these haveparticular disadvantages. For example, graphite is extremely messy andtends to produce a great deal of carbon dust under almost anycircumstances. Thus, susceptors formed of solid graphite heat thesample, but raise a number of additional handling problems that makeit's use disadvantageous.

[0041] Similarly, solid silicon carbide likewise responds to microwaveradiation. Silicon carbide is, however, notoriously difficult tomanufacture in large wafer sizes and its degree of response to microwaveradiation is relatively high, meaning that a susceptor formed entirelyof silicon carbide has to be extremely thin to avoid overheatingsensitive samples while still becoming heated in a reasonably shortperiod of time. Additionally, temperature control of susceptors formedentirely of silicon carbide tends to be difficult. Furthermore, thinpads of silicon carbide are difficult and expensive to manufacture andhard to handle.

[0042] By blending these materials in the manner of the presentinvention, the resulting pad is more durable, costs less to manufacture,is more easily formed into a disk, is easier to clean and reuse, andheats faster. Silicon carbide and graphite also absorb the magnetic andelectric components of electromagnetic fields slightly differently fromone another, so that their combination enhances the overall response ofthe susceptor.

[0043] Silicone rubber is a material of choice because it is generallysafe, non-toxic, non-volatile, physically easy to work with, resistantto physical and chemical degradation even at the expected operatingtemperatures, easy to mold quickly, and blends well with siliconcarbide, graphite, and garnet.

[0044] Experimental—Heating Profiles:

[0045] Round susceptor pads according to the present invention wereformed using garnet, silicon carbide, graphite, and blends of thesematerials in proportions by weight that were the same or similar tothose set forth above with respect to the silicone rubber. The pads werethen exposed to microwave radiation and the temperature of each pad wasmeasured at the edge (i.e., near the circumference) of the pad, at thecenter of the pad, and at a point midway between the center and theedge. The results were as follows (temperatures are in degreesCentigrade): Material Center Midpoint Edge Garnet 117 130 140 SiC 114 8979 Graphite 115 128 113 SiC:Garnet(1:1) 119 108 102 SiC:Garnet(4:1) 115118 110 SiC:Graphite(1:1) 112 120 116

[0046] These results demonstrate that an appropriate blend of twomicrowave-responsive materials provides more uniform and consistentheating across the susceptor pad, and that the preferred blend of SiCand graphite provided the most consistent results.

[0047] Analysis of Powdered Milk

[0048] Samples of powdered nonfat dry milk were obtained from severalsuppliers and were tested using the apparatus and method of the presentinvention. As noted above, the low moisture content of powdered milkmakes it particularly difficult to successfully analyze usingconventional microwave techniques. Each sample weighed approximately 4grams and two portions of each sample were tested. The analysis wascarried out for about 5 minutes, which provided sufficient time for theweight of each sample to reach a constant value, thus indicatingdryness. In each case the temperature was maintained at between about115° and 120° C. using the temperature control techniques and apparatusof the invention. The results are summarized in the following table inwhich the respective supplier provided the standard analysis of eachsample: Sample Microwave Analysis Standard Analysis 1 3.28 3.3 1 3.303.3 2 3.74 3.9 2 3.86 3.9 3 3.62 3.6 3 3.53 3.6 4 3.57 3.5 4 3.40 3.5 54.40 4.48 5 4.45 4.48

[0049] The results demonstrate that the invention provides a moisturecontent analysis technique that is as accurate as current standardanalysis techniques.

[0050] As other advantages of the invention, the rotation of the sample,and preferably the sample and susceptor, tends to heat the sample morequickly and more uniformly. These advantages in turn enhance thetemperature measurement, feedback, and control aspects of the apparatusand method. The thermal mass of the susceptor also helps heat the samplemore efficiently, and gives better process control because the heat lossis slow rather than rapid. The method of the invention also comprisesmeasuring the temperature of the susceptor while the susceptor isrotating on the balance pan, and then using the measured temperature tocontrol or moderate the microwave radiation sent into the cavity by thesource. An appropriate infrared temperature measuring technique is setforth in co-pending and commonly assigned U.S. patent application Ser.No. 09/156,086 incorporated earlier herein. As discussed therein, aninfrared temperature monitor is particularly preferred because it tendsto measure only heat being admitted by the sample or the sample pad.Using the present invention, the rotation and the equilibrium thatrotation brings to bear helps keep the temperature of the susceptor andthe sample the same. Thus, the invention produces a very accuratetemperature measurement, which in turn provides for very accuratetemperature control. Additionally, because microwaves are generallyconsidered to be at quite different frequencies than infrared radiation,the presence of large amounts of microwaves in the cavity does notinterfere with the infrared detection technique. Generally speaking, theprocessor 34 referred to earlier herein, can likewise handle theappropriate data from the temperature measurement, and use it tomoderate the radiation either from the source 20 or that can be allowedto travel from the source to the cavity.

[0051] In actual use, the balance is set to tare with the susceptor onthe pan. The sample is then added to the susceptor, the weight isrecorded, and then the microwaves are applied. The temperature of thesample and susceptor are monitored constantly, and the measuredtemperature is used to moderate the microwave power applied in order tokeep the sample and susceptor at a controlled temperature that isusually preselected based upon the material being dried (the generalcharacteristics of which are usually known). For example, in thepharmaceutical industry certain standardized tests must be carried outat specific temperatures in order to be valid or certified or both.

[0052] The sample and susceptor are kept at the controlled temperaturefor a fixed period of time (again based at least in part on the generalcharacteristics of the material being dried) after which the sample isreweighed and the moisture content calculated based on the weight lossand sample mass.

[0053] The invention can also preferably incorporate the techniques ofpreviously incorporated application Ser. No. 09/398,130 which improvesthe accuracy of weight measurements by adjusting apparent weight to trueweight by considering weight bias caused by air density gradients. Asset forth therein, this is preferably carried out by the steps ofmeasuring an apparent weight of a substance, concurrently measuring asurface temperature of the substance, concurrently measuring an ambientair temperature surrounding the substance, predicting buoyancy forcesacting upon the substance based on the surface temperature of thesubstance and the ambient air temperature surrounding the substance, anddetermining a true weight of the substance by correcting the apparentweight of the substance by the predicted buoyancy forces acting upon thesubstance.

[0054] The invention has been described in detail, with reference tocertain preferred embodiments, in order to enable the reader to practicethe invention without undue experimentation. A person having ordinaryskill in the art will readily recognize that many of the components andparameters may be varied or modified to a certain extent withoutdeparting from the scope and spirit of the invention. Furthermore,titles, headings, or the like are provided to enhance the reader'scomprehension of this document and should not be read as limiting thescope of the present invention.

That which is claimed is:
 1. A method of microwave assisted contentanalysis, the method comprising: applying microwave radiation to asample on a microwave-responsive sample pad while, concurrently rotatingthe sample and sample pad while the microwave radiation is beingapplied; and while, concurrently weighing the sample; and whilemeasuring the temperature of the rotating sample and pad; and moderatingthe applied microwave radiation in response to the measured temperature.2. A content analysis method according to claim 1 comprising maintaininga balance accuracy of ±10 milligrams while concurrently weighing androtating the sample under the applied microwave radiation.
 3. A contentanalysis method according to claim 1 comprising maintaining a balanceaccuracy of ±1 milligram while concurrently weighing and rotating thesample under the applied microwave radiation.
 4. A content analysismethod according to claim 1 comprising maintaining a balance accuracy of±0.1 milligram while concurrently weighing and rotating the sample underthe applied microwave radiation.
 5. A content analysis method accordingto claim 1 comprising rotating the sample and sample pad in asubstantially horizontal plane.
 6. A content analysis method accordingto claim 5 comprising rotating the sample pad at speed of rotation thatis sufficient to maintain an equilibrium temperature or equilibrium rateof temperature change in a sample thereon, while less than a speed atwhich the sample would become dislodged from the pad or at which thedesired accuracy of the balance would be reduced.
 7. A content analysismethod according to claim 6 comprising rotating the sample pad at aspeed of less than 30 rpm.
 8. A content analysis method according toclaim 6 comprising rotating the sample pad at a speed of between about 5and 20 rpm.
 9. A content analysis method according to claim 1 whereinthe step of applying microwave radiation to the sample comprises:generating microwaves from a source; transmitting the microwaves fromthe source to a waveguide in communication with the source; transmittingthe microwaves from the waveguide to a cavity that contains the sample.10. A content analysis method according to claim 1 wherein the step ofmeasuring the temperature comprises measuring the infrared radiationemitted by the sample and pad.
 11. A content analysis method accordingto claim 9 wherein the step of moderating the microwave radiationcomprises moderating the radiation produced by the microwave source. 12.A content analysis method according to claim 9 wherein the step ofmoderating the microwave radiation comprises moderating the transmissionof microwaves between the source and the cavity.
 13. A content analysismethod according to claim 1 and further comprising concurrentlymeasuring the ambient air temperature surrounding the sample andsusceptor pad; predicting buoyancy forces acting upon the sample and padbased on the surface temperature of the sample and pad and the ambientair temperature surrounding the sample and pad; and determining a trueweight of the substance by correcting the apparent weight of thesubstance by the predicted buoyancy forces acting upon the substance.14. An apparatus for microwave-assisted content analysis, said apparatuscomprising: a source of microwave radiation; a cavity in microwavecommunication with said source; a balance with at least its pan in saidcavity; means for rotating said balance pan within said cavity whileconcurrently measuring the weight of a sample on said pan; means formeasuring the temperature of the sample; and means for moderating themicrowave radiation in response to the measured temperature.
 15. Amicrowave apparatus according to claim 14 wherein the portion of saidbalance in said cavity does not interfere with the propagation or modesof microwaves in said cavity.
 16. A microwave apparatus according toclaim 14 wherein said microwave source is selected from the groupconsisting of magnetrons, klystrons, solid state devices, and switchingpower supplies.
 17. A microwave apparatus according to claim 14 andfurther comprising a waveguide between said source and said cavity, andin microwave communication with both said source and said cavity.
 18. Amicrowave apparatus according to claim 14 wherein said balance comprisesa force restoration balance.
 19. A microwave apparatus according toclaim 14 wherein said balance maintains an accuracy within 10 milligramswhile said balance pan is rotating.
 20. A microwave apparatus accordingto claim 14 wherein said balance maintains an accuracy within 1milligram while said balance pan is rotating.
 21. A microwave apparatusaccording to claim 14 wherein said balance maintains an accuracy within0.1 milligram while said balance pan is rotating.
 22. A microwaveapparatus according to claim 14 wherein said rotation means can providea speed of rotation from zero to about 30 rpm.
 23. A microwave apparatusaccording to claim 14 wherein said temperature measurement meanscomprises an infrared detector.
 24. A microwave apparatus according toclaim 14 wherein said moderating means comprises means for moderatingthe microwaves produced by said source.
 25. A microwave apparatusaccording to claim 14 wherein said moderating means comprises means formoderating the transmission of microwaves between said source and saidcavity.
 26. A microwave apparatus according to claim 14 and furthercomprising: means for concurrently measuring the ambient air temperaturesurrounding the sample and susceptor pad; means for predicting buoyancyforces acting upon the sample and pad based on the surface temperatureof the sample and pad and the ambient air temperature surrounding thesample and pad; and means for determining a true weight of the substanceby correcting the apparent weight of the substance by the predictedbuoyancy forces acting upon the substance.
 27. A susceptor for microwaveassisted content analysis, said susceptor comprising: a pad formed ofblend of a first material dispersed in a second material; said firstmaterial being a material that generates heat in response to microwaveradiation; and said second material being nonresponsive to microwaveradiation, but conductively heated by said first material so thatexposing said susceptor to microwave radiation evenly heats said pad anda sample on said pad.
 28. A susceptor according to claim 27 wherein saidblend includes a third material different from said first material, andthat also generates heat in response to microwave radiation, but thatresponds differently to microwave radiation than does said firstmaterial.
 29. A susceptor according to claim 28 wherein said first andthird materials are selected from the group consisting of siliconcarbide, graphite and garnet.
 30. A susceptor according to claim 29wherein said second material comprises silicone rubber.
 31. A susceptoraccording to claim 28 wherein said first and third materials are presentin said susceptor in a minimum amount that is sufficient to provide saidsusceptor with a rapid and uniform temperature response to microwaveradiation.
 32. A susceptor according to claim 28 wherein said firstmaterial and third materials are present in an amount by weight ofbetween about 8 and 14 percent each, with the remainder being saidnonresponsive material.
 33. A susceptor according to claim 32 comprisingabout 12 percent by weight silicon carbide, about 10 percent by weightgraphite and about 78 percent by weight silicone rubber.